A passive optical network (PON) is a point-to-multipoint optical tree, with upstream capacity shared according to allocations of time by an optical line terminal (OLT) that acts as the node at the root of the optical tree. During each optical network unit's (ONU) allocated time, also referred to as a burst period, the ONU transmits a burst of modulated light that contains whatever information that ONU wishes to send upstream. The modulation format may convey information by way of phase or frequency modulation of one or more fixed power optical carriers or by way of a multiplicity of optical power levels. In all such modulation arrangements, the ONU is allowed to transmit optical power in excess of a designated minimum threshold only during its allocated burst period. By way of example, and not as a limitation, this description assumes a modulation arrangement comprising two optical power levels, designated a logical zero (low optical power) and a logical one (high optical power). Between bursts, the ONU may be required to turn off its optical transmitting device, such that it transmits zero optical power. Standards allow for turn-on and turn-off time at the beginning and end of a burst, respectively, during which an ONU may emit non-zero optical power, but not to exceed the specified minimum threshold. Before the turn-on time begins, and after the turn-off time ends, the ONU is required to transmit effectively no optical power.
The OLT controls the size and sequence of bursts from the plurality of ONUs through a so-called bandwidth map, which it broadcasts to all ONUs from time to time. When constructing its bandwidth map, the OLT reserves an interval, known as an inter-burst gap, a guard time, or a guard interval, between adjacent burst periods, such that the bursts will not overlap under normal circumstances. It is therefore expected that each pair of bursts be separated by a dark interval, during which no ONU should be transmitting optical power, at least a level of power below the logical zero threshold. The minimum or guaranteed dark interval is equal to the inter-burst gap specified by the bandwidth map, reduced by a tolerance for turn-on and turn-off time and by allowances for timing uncertainties in the ONUs' responses. The guard time and tolerances are bounded by standards. For example, the International Telecommunications Union recommends a minimum guard period of 64 bit times in “G.987.2, 10-Gigabit-capable Passive Optical Networks (XG-PON): Physical Media Dependent (PMD) Layer Specification” (January 2010). In this exemplary implementation, the accumulated tolerances may reduce the guaranteed dark interval to as few as 16 bit times.
While PON technology offers substantial economic benefit, it suffers from the risk that a defective ONU, a node at one of the leaves of the tree, can destroy some or all traffic on the PON by transmitting light continuously or at unauthorized times. Such an ONU is commonly known as a rogue ONU. The multipoint-to-point nature of the upstream PON makes it difficult to identify and isolate a rogue ONU.
Existing solutions include means by which an ONU can monitor its own behavior. For example, a watchdog timer can shut down an ONU if its transmitter remains on continuously. There are also existing messages from the OLT to direct the ONU to shut down, which may be effective if the ONU is able to receive and respond to such messages.
Algorithmic techniques have been proposed to identify and isolate a rogue ONU. Existing algorithms involve, for example, disabling all ONUs and then re-enabling one at a time to search for the rogue ONU; alternatively, disabling one ONU at a time to search for the rogue, or rearranging the burst allocation sequence. Because these techniques disrupt service, they are generally not invoked automatically, which means that a rogue ONU may itself disrupt service for an extended period of time before diagnostics are initiated by maintenance personnel.
Existing art provides no way to detect the presence of a rogue ONU other than deduction based on generalized PON failure or erratic PON performance, the latter of which may be recognized only by customer complaints. It would facilitate the diagnosis process to have an indication that a rogue ONU is present.